Method of casting by directional solidification



2 Sheets-Sheet l B. .1. PIEARCEY Dec. 23, 1969 METHOD OF CASTING BYDIRECTIONAL SOLIDIFICATION Original Filed July 16, 1965 Maw/M gullllliiiMETHOD OF CASTING BY DIRECTIONAL SOLIDIFICATION 2 Sheets-Sheet 2Original Filed July 16, 1965 United States Patent 3,485,291 METIIUD 0FCASTING BY DIRECTIONAL SOLIDIFICATION Barry I. Piearcey, Cheshire,Conn., assignor to United Aircraft Corporation, East Hartford, Conn., acorporation of Delaware Original application July 16, 1965, Ser. No.472,611. Divided and this application June 7, 1968, Ser. No. 753,308

Int. Cl. B2211 15/04 U.S. Cl. 164-127 2 Claims ABSTRACT OF THEDISCLOSURE This invention relates to a chill plate for use indirectionally solidified castings. The chill plate is used with atemperature-controlled shell mold which heats the casting material andestablishes a unidirectional temperature gradient between the materialand the chill plate. The surface of the chill plate in contact with thematerial is roughened to improve heat transfer to the plate and topromote columnar grain growth within the material. This application isdirected to the method of casting with this chill plate.

This is a division of the Piearcey application, Ser. No. 472,611, filedJuly 16, 1965, now abandoned.

This invention relates to a chill plate for use in making castingshaving a controlled grain structure and to a process and mold for usetherewith.

The copending application of VerSnyder Ser. No. 361,- 323, filed Apr.17, 1964 now Patent No. 3,260,505, is sued July 12, 1966, and assignedto the same assignee as the present application is directed to a castturbine blade or vane having a columnar grained structure. One featureof this invention is a chill plate construction that will facilitate theproduction of this type of casting. Another feature is a surfacetreatment of the chill plate to improve the start and continued growthof the columnar grains.

The growth of the columnar grains is promoted by rapid heat removalthrough the chill plate and a feature of the invention is a roughenedchill plate surface thereby increasing the surface area of the plate incontact with the metal being cast. It has been found that thecrystalline growth in the desired direction can be substantiallyimproved by more rapid heat dissipation into the chill plate.

Other features and advantages will be apparent from the specificationand claims, and from the accompanying drawings which illustrate anembodiment of the invention.

In the drawings:

FIGURE 1 is a sectional view through a mold embodying the invention.

FIGURE 2 is an enlarged fragmentary vertical sectional view through achill plate having a smooth surface, and part of a casting thereon.

FIGURE 3 is an enlarged view similar to FIGURE 2 but in which the chillplate has a roughened surface.

FIGURE 4 is an enlarged sectional view of a smaller scale of a portionof the surface of a modified form of chill plate.

FIGURE 5 is a plan view of a modified form of chill plate.

The invention is for use primarily in directionally solidified castingas described in the above-identified application of VerSnyder, Ser. No.361,323. The purpose of that invention is to produce such elements asturbine blades or vanes such that the crystallization will extendlongitudinally of the element and with the grain boundaries alsoextending in this same direction.

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Referring first to FIGURE 1, the mold is shown as a shell mold 2positioned with bottom opening 4 thereon on a chill plate 6. The shellmold is arranged to produce a plurality of turbine blades or vanes atone time and to this end incorporates a plurality of vane-shapedrecesses 7 in parallel vertical arrangement. These recesses communicateat the lower ends with growth area recesses 8 and at their upper endswith top filling areas 10. These latter connect through lateral passages12 with the central pouring sprue 14 by which all the recesses 7 can befilled simultaneously.

The mold, when completed and ready to be used for casting structuralparts, is positioned within a ring or cylinder of heating elements 16 asdescribed in the copending application of Barrow and Sink, Ser. No.472,- 644 filed July 16, 1965, now Patent No. 3,405,220, issued Oct. 8,1968, and having the same assignee as this application. These heatingelements provide for heating the mold to a temperature above the meltingtemperature of the alloy and are preferably arranged to be successivelyturned off during the casting process.

According to the present invention the chill plate 6 has a roughenedsurface 18 contacting with the mold and with the molten metal when it ispoured therein. The roughness may be obtained by forming a plurality ofgrooves 20 therein as in FIGURE 4 or by knurling 22 as in FIGURE 5 toprovide a substantially larger area of contact between the chill plateand the molten metal. The grooves 20 of FIGURE 4 may be rectilineargrooves parallel to one another or may be annular or spiral grooves toproduce substantially the configuration shown. It has been found thatthe grooves should be less than one-tenth of an inch in depth and thespacing should be close enough together to produce an effectivelygreater contact surface. With a depth of groove as described the moldcontact with the chill plate requires no particular sealing arrangementsince the immediate cooling of the molten metal as it contacts the chillplate effectively seals any of the small spaces that might exist.

The showing of FIGURES 2 and 3 shows graphically the improvement in caststructures with the roughened chill plate. These views are about fiftytimes normal size. In FIGURE 2 it is apparent that the grain growth israndom rather than directional and no directionally oriented graingrowth becomes established for a substantial distance from the chillplate. However, with a chill plate as in FIGURE 3, the columnar graingrowth begins at the peak or top surfaces of the ridges in the chillplate and a direction substantially at right angles to the plate is wellestablished almost immediately.

In addition to the directional grain growth being well established ithas been found that the roughened chill plate surface improvesmaterially the heat transfer from the melted alloy to the chill plate sothat solidification occurs at a substantially faster rate than is thecase with a smooth plate. It has been found that the desirableproperties in the finished part are best obtained with rapidsolidification and the roughened plate contributes to this effect.

It is well known that the rate of thermal conductivity is much less inthe well known alloys used in high temperature environments such as gasturbine vanes and blades, than in other simpler metals that would beused in chill plates. The ability of the chill plate to remove heatfaster by its roughened surface helps to maintain a higher temperaturegradient in the article being cast and thus increases to an optimum theeffective rate of heat transfer lengthwise of the blade or vane andthence into the chill plate.

With the nickel or cobalt base superalloys which have beenunidirectionally solidified, it has been found that the 001 direction ofgrowth produces the crystalline orientation for optimum properties.During solidification the dendrite crystals grow fastest in the 001direction and under the temperature gradient resulting from a heatedmold and a chill plate this growth is substantially perpendicular to thechill plate surface. This temperature gradient is effectively measuredby the increase in surface area resulting from the grooves formed in thesurface. The roughness also serves to provide a more effectivemechanical joint so that casting and chill plate stay in better contactduring the solidification process.

The grooves increase the number of dendrites per unit area by a factorof more than two. This promotes the possibility of obtaining anorientation closer to 001 Further, as shown in FIGURE 3, a greaterproportion of the dendrites are oriented substantially perpendicularlyto the chill plate than where the surface of the chill plate is smooth.

The grooved chill plate by measuring the rate of cooling has effectivelyreduced the size of the carbides in the alloy. These zone carbidesaffect the machinability of the alloy, particularly in electrochemicalmachining of the alloy and thus the parts produced by the use of theroughened chill plate has improved machinability substantially.

It is to be understood that the invention is not limited to the specificembodiment herein illustrated and described but may be used in otherways without departure from its spirit as defined by the followingclaims.

I claim:

1. In casting a high temperature nickel or cobalt base 4 alloy bydirectional solidification to produce columnar grains, the steps ofproviding a controlled substantially uniform roughness in the topsurface of a chill plate member, mounting a shell mold structure havinga pouring spout at its top end and an open bottom end on said platemember so that said roughness forms part of the mold cavity, heating themold to a temperature above the melting point of the alloy while keepingthe plate member cooled to a relatively low temperature which isconsiderably less than the melting point of the plate member material,pouring the alloy into the mold against the chill plate member therebycausing the start of growth of the grains on the high points of theroughness and continuing to cool the chill plate member as the remainderof the mold is gradually cooled.

2. In the method of casting as claimed in claim 1 in which thecontrolled roughness is in the form of uniformly spaced groovesproviding a depth of roughness less than substantially A of an inch.

References Cited UNITED STATES PATENTS 2,951,272 9/1960 Kiesler 164-127X 3,248,764 5/1966 Chandley 164-427 FOREIGN PATENTS 1,183,121 11/1959France.

0 I. SPENCER OVERHOLSER, Primary Examiner R. D. BALDWIN, AssistantExaminer

